The positive photoresist of the present invention is coated on a substrate, such as semiconductor wafers, glass, ceramics, or metals, by spin coating or roller coating, at a thickness of approximately from 0.5 to 3 .mu.m. After thus coated, this is heated, dried and exposed to, e.g., ultraviolet rays, via an exposure mask, to form a circuit pattern or the like on the substrate. After thus exposed, this is then developed to give a positive image on the substrate.
In addition, the substrate having the thus-formed positive image thereon is etched, using the positive image as a mask, to have a pattern thereon. One typical technical filed to which the photoresist is applied includes manufacture of semiconductor devices such as IC, manufacture of circuit substrates for liquid crystal displays, thermal heads, and other photofabrication processes.
Positive photoresist compositions generally comprise an alkali-soluble resin and, as a photosensitive substance, a naphthoquinonediazide compound. For instance, compositions comprising novolak phenolic resins and substituted naphthoquinonediazide compounds are disclosed in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470. As the most typical compositions, examples comprising cresol-formaldehyde novolak resins and trihydroxybenzophenone-1,2-naphthoquinonediazido sulfonates are disclosed in L. F. Thompson, Introduction to Microlithography (published by ACS, No. 219, pp. 112-121).
Novolak resin as a binder is soluble in an aqueous alkaline solution, without being swollen. In addition, it is highly resistant to plasma etching, when formed image is used as a mask for etching. Therefore, the novolak resins are especially advantageous as binder in this use. Naphthoquinonediazide compound used for a photosensitive substance acts by itself as a dissolution inhibitor capable of reducing the solubility of novolak resins in alkalies but, when exposed to light, specifically decomposes to give an alkali-soluble substance which rather increases the solubility of novolak resins in alkalies. Due to such significant variation in their properties relative to exposure to light, naphthoquinonediazide compounds are especially useful as photosensitive substances in positive photoresists.
Accordingly, many positive photoresists containing novolak resins and naphthoquinonediazide-based photosensitive components have heretofore been developed and put to practical use. These have produced sufficient results in forming line patterns on a level of approximately from 1.5 .mu.m to 2 .mu.m.
However, the degree of integration in integrated circuits is desired to be increased more and more in these days, and it has become necessary to produce ultra-fine patterns having a line width of 1 .mu.m or less in the production of semiconductor substrates for ultra-LSI. In such use, photoresists having high resolution and high sensitivity are needed, in order that they can correctly transcribe the profiles of exposure masks with high reproduction accuracy to reproduce correct patterns at high reproducibility. However, the above-mentioned conventional positive photoresists cannot satisfy these requirements.